1. Field of the Invention
The invention relates to photochromic, electrochromic, photoelectrochromic and photovoltaic devices. As used herein, photochromic generally refers to a change in color from light absorption, electrochromic refers to an electrically induced change in color, photoelectrochromic refers to a change in color produced by photogenerated charge carriers and photovoltaic refers to generating a voltage by light absorption.
2. Background Art and Related Disclosures
Both photochromic materials which change color upon absorption of light, and electrochromic materials which change color in response to an electrically induced change in oxidation state, have shown great promise for future, so called "smart" windows and information display applications. Both of these classes of optically reversible switching materials offer dynamic control of visible light levels and solar radiation with long memory times, and thus the ability to control illumination levels as well as glare, heat gain and heat loss.
This ability to control the transfer of heat, cold and light through glass has a very high utility, energy saving and commercial potential and, therefore, the availability of materials possessing a combination of these properties and having these qualities would be very valuable. However, until now, materials having both of these properties, that is materials which change color upon absorption of light and also in response to an electrically induced change in oxidation state, are not in any practical sense available.
Although there are many metal oxides, polymers and other materials which exhibit some or all of those properties, and in spite of a large number of studies on these materials in many configurations, there still remain fundamental problems that have prevented a large-scale fabrication and commercialization of photochromic and electrochromic devices. These problems include inadequate reversibility, lack of long-term stability and high costs of fabrication.
A traditional electrochromic device usually consists of a number of sequentially deposited layers, including the active electrode, electrolyte, ion-storage layer and counter electrode (or the electrochromic materials may be dispersed in a solid or semi-solid electrolyte). These layers must be deposited on a conducting substrate. In these instances, the oxidation state of the active material and consequently the color of the device is controlled by the potential between the active electrode and counter electrode.
A self-powered "smart" electrochromic window which could control its own transparency is described in Nature, 383:608 (1996). This system combined two separate electrodes: a dye-sensitized semiconductor TiO.sub.2 photoelectrode and a WO.sub.3 electrochromic electrode. The electrochromic effect of this composite system is ascribed entirely to the transfer of photogenerated charge between the electrodes through an external circuit. The TiO.sub.2 electrode drives the WO.sub.3 electrode to change from transparent to dark blue (Li.sub.x WO.sub.3) under illumination. The electrons travel between the electrodes through an external circuit and the resulting negative charge is compensated by positively charged lithium ions from an electrolyte phase, which intercalate into the layered WO.sub.3 structure. This structure is similar to the traditional electrochromic device. However, the system is complicated and unstable.
A photoelectrochromic effect in WO.sub.3 colloids with steady-state UV photolysis in the presence of oxalic acid as a hole scavenger was described in J. Phys. Chem., 97:11064 (1993). The photoelectrochromic characteristics of thin Prussian blue (PB) on both single crystal and polycrystalline TiO.sub.2 electrodes was described in J. Electrochem. Soc., 130:249 (1983). It was found that illumination of the composite electrode led to the oxidation of the colorless Prussian white (PW) film to Prussian blue at potentials at least 500 mV lower than the PW/PB redox potential for a modified Pt electrode.
However, none of these described devices and structures have been advantageously utilized for fabrication of a useful device as they are typically costly, laborious to fabricate and lack the properties required for wide use and distribution.
U.S. Pat. No. 4,599,614 issued Jul. 8, 1986 to DeBerry et al. for Photoelectrochromic Display and U.S. Pat. No. 5,604,626 issued Feb. 18, 1997 to Teowee et al. for Photochromic Devices are illustrative of the prior art.
All patents, patent applications, published articles and abstracts referred to in the specification are hereby incorporated by reference in their entirety.